Some of the work presented herein was supported by a grant from the National Institutes of Health (CA79553). The United States government may, therefore, have certain rights in the invention.
Polyunsaturated fatty acids (PUFAs) are fatty acids having 18 or more carbon atoms and two or more double bonds. They can be classified into two groups, n-6 or n-3, depending on the position (n) of the double bond nearest the methyl end of the fatty acid (Gill and Valivety, Trends Biotechnol. 15:401–409, 1997; Broun et al., Annu. Rev. Nutr. 19:197–216, 1999; Napier et al., Curr. Opin. Plant Biol. 2:123–127, 1999). The n-6 and n-3 PUFAs are synthesized through an alternating series of desaturations and elongations beginning with either linoleic acid (LA, 18:2n6) or α-linolenic acid (ALA, 18:3n3), respectively (Gill and Valivety, supra; Broun et al., supra; Napier et al., supra). The major end point of the n-6 pathway in mammals is arachidonic acid (AA, 20:4n6) and major end points of the n-3 pathway are eicosapentaenoic acid (EPA, 20:5n3) and docosahexaenoic acid (DHA, 22:6n3).
An important class of enzymes involved in the synthesis of PUFAs is the class of fatty acid desaturases. These enzymes introduce double bonds into the hydrocarbon chain at positions determined by the enzyme's specificity. Although, in most cases, animals contain the enzymatic activity to convert LA (18:2n6) and ALA (18:3n3) to longer-chain PUFA (where the rate of conversion is limiting), they lack the 12- and 15-desaturase activities necessary to synthesize the precursor (parent) PUFA, LA and ALA (Knutzon et al., J. Biol. Chem. 273:29360–29366, 1998). Furthermore, the n-3 and n-6 PUFA are not interconvertible in manmalian cells (Goodnight et al., Blood 58: 880–885, 1981). Thus, both LA and ALA and their elongation, desaturation products are considered essential fatty acids in the human diet. The PUFA composition of mammalian cell membranes is, to a great extent, dependent on dietary intake (Clandinin et al., Can. J. Physiol. Pharmacol. 63:546–556, 1985; McLennan et al., Am. Heart J. 116:709–717, 1988).
To the contrary, some plants and microorganisms are able to synthesize n-3 fatty acids such as ALA (18:3n-3) because they have membrane-bound 12- and 15-(n-3) desaturases that act on glycerolipid substrates in both the plastid and endoplasmic reticulum (Browse and Somerville, Annu. Rev. Plant Physiol. Plant Mol. Biol. 42: 467–506, 1991). Genetic techniques have led to the identification of the genes encoding the 12- and 15-desaturases from Arabidopsis thaliana and other higher plant species (Okuley et al., Plant Cell 6:147–158, 1994; Arondel et al., Science 258:1353–1355, 1992). Recently, a fat-1 gene encoding an n-3 fatty acid desaturase was cloned from Caenorhabditis elegans (Spychalla et al., Proc. Natl. Acad. Sci. USA 94:1142–1147, 1997; see also U.S. Pat. No. 6,194,167).